As I am working from home 95% of the time, I also often log on to the Teams meeting with my camera off and my audio muted, as I am stomping around the kitchen in my PJs and cooking breakfast, might have even forgotten about the meeting until a notification pops up on my screen. Meaning, that neither is logging on to a Teams call without audio/video odd to me (after all she was walking her dog at the time and her background noise and moving video would be distracting to others), nor is the idea that someone other than her could have pressed "join" on a Teams event notification outlandish to me - a stranger, unlikely, but someone that knew her routines well, yeah, totally could have.
If it was a longer meeting (that meetings with loads of people tend to be - and you are more likely to just silently listen in on these), IMO it could have even not been over yet by the time her phone was found! Or, as others sign off at the end, how long do you stay connected to the call after a meeting if you do not sign off? I know you can accidentally log on again to random old meetings if you klick on the wrong place (whoops), so it's not like they are disabled after the meeting time in the calendar is over or if you're the only person there. Any input, fellow Teams users?